Everyone travels forward in time at a speed of one hour per hour. There are a few exceptions to this speed but basically it is a constant.
I would guess that your not talking about that but instead the science fictional time travel. Here is my theories on it.
If it is possible to travel back in time, one of two thing would happen.
1) You create a tangent along the timeline that has no effect on the future you came from. For the people along that timeline, seeing that it would have no effect it would not be any less safe. Clearly if time travel ever does exist and they existed along our timeline then we would have heard about it. This means that if it does happen in the future they only have effected other timelines not ours. There are far too many paradoxes for it to happen along ours.
2) Going back in time wold instantly cause a black hole destroying the world if not the universe.
Lets say you travel back in time one year at exactly midnight on New Years Eve 2015. This means at midnight on New Years Eve 2014 there are two of you. The you that just went back in time and the one that will. This second you will also travel back in time to the exact place and time as you did. If that was the end, your mass would instantly double at the moment you arrived in the past. Problem is, there would still exist another you that has not traveled back in time yet meaning that your mass will not just double but instantly become infinite. This is how you instantly become a black hole that destroys the earth and possible everything. Obviously if it was possible to travel back in time, even if you sent something simple like a tennis ball its mass would instantly become infinite. This means that if it has been attempted then it would simply destroy the tangent. The other idea would be that if it is attempted in the future and does not create a tangent then it would destroy our own existence.
Did you ever think that there is a possibility that when you go back in time you change something, for example: you stop Hitler. What happens next? There are 10 trillion people, your family doesn't exist any more etc. Maybe you do go back in time and bring back an unknown virus. Than there is 2 trillion people left. Time travel can also kill you just like that. Maybe your machine goes to early humans but you get reversed, instead of growing you shrink till your nothing ( opposite of life ). Early humans might use the machine and go to the future, many families wont exist any more because there is a hole in the family tree. Time machines are dangerous to everything.